The problem is most likely with your geometry manager arguments. However you are not likely to get any useful help unless you show the code where your problem lies. And please remember to enclose it in <code></code> tags.

use strict;
use warnings;
use Tk;

my $mw = MainWindow->new();
$mw->Canvas(-bg => 'red',
            -highlightthickness => 0,
        )->pack;
$mw->Canvas(-bg => 'green',
            -highlightthickness => 0,
        )->pack;
$mw->MainLoop;
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Thank you all for reading and replying. Christoph solve my problem. I couldn't believe I didn't see the highlightthickness option.